How can I see who has not signed up yet?

I am not sure if I understand your query correctly. Did you send the form to a list of users and wanted to compare with the submissions? If so, unfortunately, there is no inbuilt feature to achieve this. However, you may export the submissions to Excel sheet and you may run a report to compare the existing list. 

If you are referring to something different, please provide us with more information. We will be happy to assist you.